Known in the art are methods to prepare .alpha.-substituted .omega.-hydroperfluoroalkanes, for example, a method for the preparation of dihydroperfluoroalkanes which includes photoinitiated chlorination of .alpha., .alpha., .omega.-trihydroperfluoroalcohols (telomeric alcohols) of the general formula: H(CF.sub.2 CF.sub.2).sub.n CH.sub.2 OH wherein n=1-5 (Brace Neal. O, J. Org. Chem., 1961, 26, No. 10, pp. 4005-4010). The reaction process proceeds at a temperature of 10.degree. to 30.degree. C. for 2-7 hours to form the corresponding aldehydes that are then converted into dihydroperfluoroalkanes by treatment of them with 50% aqueous solution of potassium hydroxide.
This method has proved, however, faulty for its complicated production process associated with the employment of UV light sources and an aggressive and toxic chlorine medium, and the necessity of conducting the process in two stages. The method has a low conversion (10-25%) of the starting materials and a sufficiently low selectivity of the reaction yield (38-68%) of the desired products in terms of the amount of the converted alcohol (yields of the desired products are less than 17% based on alcohol to be charged). It should be also noted that the formation of aldehydes is accompanied by evolution of HCL gas, and, therefor, further neutralization step should be added. For the reasons as stated above this method failed to find industrial use.
Known in the art are the processes to prepare .omega.-hydroperfluorocarboxylic acids, which include the oxidation reaction of .alpha., .alpha., .omega.-trihydroperfluoroalcohols (telomeric alcohols) with various oxidizing agents.
It has been proposed a method for the preparation of .omega.-hydroperfluorocarboxylic acids of the formula H(CF.sub.2 CF.sub.2).sub.n COOH wherein n=1-4, which includes the oxidation reaction of telomeric alcohols with nitric oxides (see, SU, A, No. 314748). The oxidation reaction is carried out at a temperature of 350.degree. to 400.degree. C. with 4 moles of nitrogen dioxide used for 1 mol of the initial alcohol. The yields of .omega.-hydroperfluorocarboxylic acids having odd number of carbon atoms are 43.4% for .omega.-hydroperfluorovaleric acid and 48.0% for .omega.-hydroperfluoroenanthic acid. Thus, this method does not ensure high yields of the desired products. In addition, the oxidation reaction is carried out at high temperatures, and the treatment of the resulting aggressive mixture consisting of .omega.-hydroperluorocarboxylic acids, nitric oxides, hydrofluoric acid and nitrogen-containing acids requires special corrosion-resistant equipment.
Further, there is proposed a method for the preparation of .omega.-hydroperfluorocarboxylic acids of the general formula H(CF.sub.2 CF.sub.2).sub.n COOH wherein n=3-6, which includes oxidizing telomeric alcohols with potassium permanganate in glacial acetic acid at a temperature of 50.degree. to 105.degree. C. (see, U.S. Pat. No. 3,423,417; U.S. Pat. No. 3,514,322 and U.S. Pat. No. 2,559,629). In order to recover the desired products, the reaction mixture, according to the invention, is filtered to remove manganese dioxide, and alternatively, treated with sulfur dioxide to convert MnO.sub.2 into manganese sulfate. Then, glacial acetic acid is removed by evaporation, the residue is acidified with sulfuric acid and extracted with ether. The ether extract is distilled off, and the acids is treated by recrystallization.
This method is faulty for its complicated production process and a large quantity of manganese-containing by-products. Moreover, the method requires additional treatment steps to remove compounds of heavy metals (manganese) from industrial effluents.